询问对方在大学里主修的专业，你可以使用以下几种表达方式：

What's your major？

What do you major in？

In which department do you study？

What's your specialty？

What field are you specialized in？

What are you studying？

What are you majoring in？

What's your field of study？

What's your degree in？

What's your area of expertise？

这些表达方式都可以有效地询问对方在大学里主修的专业。根据具体的语境和需要，你可以选择最适合的一种或几种来表达你的问题。